[Support] Don't print stacktrace if DbgHelp.dll hasn't been loaded yet

On Windows certain function from `Signals.h` require that `DbgHelp.dll` is loaded. This typically happens when the main program calls `llvm::InitLLVM`, however in some cases main program doesn't do that (e.g. when the application is using LLDB via `liblldb.dll`). This patch adds a safe guard to prevent crashes. More discussion in
https://reviews.llvm.org/D119009.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D119181